Hair structure

Hair is a keratinized thread-like appendage of the skin, 0.005-0.6 mm thick and from a few millimeters to one and a half meters long. The length and thickness of hair depends on many factors: race and gender, age, location, etc.

Hair follicles are located everywhere on the surface of the human body, with the exception of some anatomical areas. Thus, hair is absent on the palms and soles, the lateral and palmar surfaces of the fingers, the red border of the lips, the head of the penis, the clitoris, the labia minora and the inner surface of the labia majora.

There are different types of hair depending on their location, length, thickness, degree of pigmentation, and the presence or absence of medulla. As for the division of hair into types, or species, there is still no single classification. In the Russian dermatological and morphological school, it is customary to distinguish three types: long, bristly and vellus.

Long - these are thick, long, pigmented hairs that cover the scalp, pubic area, armpits after puberty. In men, long hair grows in the beard, moustache and other areas of the skin.

Bristly hairs are also thick and pigmented, but unlike long hairs, they are significantly shorter. Hairs of this type form eyebrows, eyelashes, are found in the external auditory canal and in the vestibule of the nasal cavity. Both long and bristly hairs contain medulla.

Vellus hair is the most numerous, thin, short, colorless hair, covering almost all areas of the skin. Western European and American dermatological schools adhere to a slightly different division of hair: they distinguish two main types - vellus and terminal. Vellus hair is soft, without medulla, rarely pigmented and rarely reaching a length of more than 2 cm. Terminal hair is characterized as coarser, longer, often pigmented and having medulla. At the same time, it is indicated that there are a large number of intermediate types. It is known that from the same hair follicle throughout life, under the influence of many exogenous and endogenous factors, hair of different types can grow. If in the prepubertal period terminal hair is limited only to the scalp, eyebrows and eyelashes, then after the end of puberty their localization expands, which is due to the activity of sex hormones. Under the influence of androgens during puberty, as well as with various endocrine diseases, vellus hair can transform into long hair. Particularly pronounced sensitivity to androgens is characterized by vellus hair in the area of the upper lip, chin, areola of the nipples of the mammary glands and located along the midline of the abdomen below the navel.

In addition to hormonal influences, the development of terminal hair is influenced by constitutional and racial factors, which are genetically determined.

Hair color depends on the activity of melanocytes and is determined by two pigments: yellow-red pheomelanin and black-brown eumelanin. Their biosynthesis occurs under the influence of genetic predisposition and the endocrine system. Hair color options depend on the combination of both pigments. Thus, black hair contains more eumelanin, and light hair contains more pheomelanin. Red hair in subjects with white skin contains only pheomelanin. The color of light brown hair is determined by a separate gene of varying expressivity.

Hair structure is also genetically determined and largely depends on race. The most noticeable differences between races are noted on the scalp. Thus, the Mongoloid race is characterized by coarse, straight hair; the Negroid race is characterized by coarse, significantly curled (spiral and "woolly") hair; the Caucasoid race is characterized by soft, slightly curly wavy hair. These hair types have different shapes on the cross-section.

Hair consists of a shaft protruding above the skin level and a root located in the hair follicle. Each follicle is a cylindrical epithelial formation (a kind of "invagination"), resembling a stocking and immersed in the depths of the dermis and hypodermis. The hair follicle is braided with a connective tissue hair follicle, consisting of an inner and outer layer. In the inner layer, the fibers are arranged circularly, and in the outer layer - longitudinally. Near the surface of the skin, the hair follicle forms an expansion called a funnel. The duct of the sebaceous gland (on all areas of the skin) flows into the funnel of the follicle, as well as the apocrine sweat gland (in the area of the armpits, areolas of the nipples of the glands on the chest, perianally, perigenitally, etc.). At the end of the follicle there is an extension - the hair bulb, into which the connective tissue hair papilla grows with a large number of blood vessels that provide nutrition to the hair bulb. The epithelial cells of the bulb are cambial elements that ensure hair growth. They actively divide and, shifting, differentiate and form (depending on the position in the bulb) cells of different types, which undergo keratinization and participate in the formation of various parts of the hair, as well as its internal root sheath. The bulb also contains melanocytes, which determine hair pigmentation, as well as nerve endings.

The hair medulla is formed by the cells of the central part of the bulb. It consists of weakly pigmented, vacuolated cells lying like coin columns and containing oxyphilic granules of trichohyalin, the precursor of the horny substance, in the cytoplasm. The cells of the medulla become completely keratinized only at the level of the sebaceous glands.

The cortex of the hair is formed by the middle part of the hair follicle. It is located around the medulla and consists of flattened spindle-shaped cells that quickly keratinize, filling with hard keratin.

The hair cuticle is formed by the outer edge of the middle part of the bulb; it surrounds the cortex and consists of cells that transform into horny scales containing hard keratin. They overlap each other in a tile-like manner with their protruding edges facing upward. It is the tight closure of the horny scales that provides the natural shine of the hair surface and prevents its excessive moisture or loss of moisture. The tight closure of the scales is ensured by the double lipid layers between them, between which there are hydrophilic substances, in particular, ceramides.

The internal epithelial sheath is formed by the peripheral part of the bulb and surrounds the hair root to the level of the sebaceous gland ducts, where it disappears. It includes three layers, clearly distinguishable only near the bulb and merging above into a single horny layer (from the inside to the outside):

• cuticle of the internal epithelial sheath - similar to the hair cuticle, its scales contain soft keratin. They are directed with their protruding edges downwards and are intertwined with the scales of the hair cuticle;
• the internal (granule-containing) Huxley layer - near the bulb, it is formed by cells containing trichohyalin granules, which, shifting upward, are filled with soft keratin and are destroyed;
• The outer (pale) layer of Henle is formed by a single row of light cubic cells that are filled with soft keratin and destroyed.

The outer epithelial sheath is a continuation of the epidermis in the follicle. It loses the stratum corneum at the level of the sebaceous glands and, thinning to 1-2 layers, merges with the bulb.

The muscle that raises the hair consists of smooth muscle elements; one end of it is woven into the hair follicle, and the other into the papillary layer of the dermis. The muscle is innervated by fibers of the autonomic nervous system.

Vellus hairs are of great importance in the formation of tactile sensitivity, acting as a kind of "sensory endings", when irritated, the contraction of the muscle that raises the hair occurs. When it contracts, the obliquely lying hair approaches a vertical position, and the skin in the area of muscle attachment is drawn in. As a result, an emphasized follicular pattern appears. This phenomenon underlies the pilomotor reflex, which characterizes the state of vegetative innervation. Hairs located in other areas also perform a number of specific functions. For example, when the hair in the vestibule of the nasal cavity is irritated, the act of sneezing occurs, and when the eyelashes are affected, the eyelids close.

[ 1 ]